Aromatic polycarbonate resins and polyester resins based the reaction of diols with diacids constitute two classes of engineering thermoplastics useful separately to prepare shaped articles by molding, extrusion, and so forth. In recent years, interest has grown in combining these polymers into blends to provide certain property profiles not usually found with use of either polymer alone.
A shortcoming associated with such polymer blends is the tendency to form a yellow or yellowish color. This may be attributable to several causes, including the thermal degradation of the polymers upon processing at elevated temperatures. To prevent or minimize this effect, various additives have been proposed, including those based on phosphorus as described in U.S. Pat. No. 3,953,539(Kawase, et al.) and British Pat. No. 1,569,296. Freitag, et al. in U.S. Pat. No. 4,366,276 disclose flame resistant compositions of aromatic polyesters, poly(estercarbonates) and small amounts of ammonium or alkali metal salts of inorganic or organic acids, e.g., phosphoric acid salts, which are said to be more color stable.